This invention relates generally to gauges and sensors used to identify downhole pressure and temperature parameters in an oil and gas well. Specifically, the invention relates to downhole sensors which provide a variable capacitance effect in response to changes in pressure of a subterranean formation.
Downhole capacitance sensors are well-known in the art and make use of the following relationship:
                    C        =                              ɛ            ⁢                                                  ⁢            A                    g                                    (                  Eq          .                                          ⁢          1                )            where “C” is capacitance, “∈” is the dielectric constant of the medium in which the sensor is encapsulated, “A” is the area of the sensor (i.e., the area represented by the opposing wall surfaces of a diaphragm and stator of the sensor), and “g” is the distance or width of the gap between opposing wall surfaces of the stator and the diaphragm. The stator and diaphragm are arranged one above the other in a horizontal plane within a protective housing having a hollow interior and an open bottom. Essentially, the available area “A” is the top surface of the diaphragm less that of a spacer or washer placed between the stator and the diaphragm, that is, π(r12−r22), where “r1” is the radius of the diaphragm and “r2” is the radius of the spacer or washer. Oil or gas enters the interior and applies pressure to an underside of the diaphragm. As the diaphragm flexes, the gap “g” between it and the stator increases (because the stator is connected to the post and moves up as the post moves) and capacitance “C” decreases, thereby indicating increased pressure. Additional explanation of the way in which this type of capacitance sensor works can be found in U.S. Pat. No. 4,125,027, to Clark, which is hereby incorporated herein by reference. A typical downhole capacitance sensor arranged in the above way exhibits a capacitance of between 25 and 40 pF. Therefore, controlling variation in the gap “g” during the assembly process is important.
An emerging size requirement for the gauges which house these capacitance sensors is that the gauge have a maximum diameter of less than 1¼ inches (3.175 cm) and, preferably, less than ¾ inches (1.905 cm). However, shrinking the size of the sensor is challenging because as the size of the sensor decreases, the area “A” decreases and, therefore, so does the capacitance “C”. As capacitance decreases, the electronic circuit used to convert capacitance to frequency has difficulty isolating the difference between the sensor's capacitance and the stray capacitance. One way around this problem is to increase the capacitance “C” by decreasing the size of the gap “d” between the stator and the diaphragm. However, reducing the gap increases the likelihood of arcing. It also increases the overall cost of manufacturing the sensor because the gap is typically about 0.003 to 0.0035 in. (0.00762 to 0.00889 cm). Maintaining this gap size requires extreme precision machining much less trying to achieve an even smaller gap size. Therefore, reducing the gap “g” is extremely difficult to achieve.
A final problem with existing downhole capacitance sensors, in addition to reducing size, is that the bonding agent between the stator and the diaphragm requires temperatures of about 900 to 1000° C. for bonding to occur. High temperatures such as this cause oxidation which then adds to the complexity and cost of manufacturing. Cleaning steps and equipment such as vacuum ovens are required.